As the technology for treating waste, such as city garbage or shredder dust, the method of waste melting treatment that thermally decomposes and burns the waste, and melts the thermal decomposition residue into slag, discharging it.
This treating method offers advantages that the waste can be thermally decomposed to be gasified, thereby allowing the combustion heat to be recovered, and after melting the thermal decomposition residue and discharging it as slag, the volume of the waste to be finally disposed can be reduced by making land-filling disposal or other. Such a melting treatment method is available in several types, and as one of them, the method is available which uses a waste gasification melting furnace of shaft furnace type, which is of vertical type.
This waste gasification melting furnace of shaft furnace type performs such a treatment as that in which the coke deposited in the lower section of the furnace is burned, and onto such coke bed at high temperature, the waste is charged to be thermally decomposed and partially oxidized for gasification, with the thermal decomposition residue being melted into slag (refer to Patent Document 1).
With the waste gasification melting furnace of shaft furnace type disclosed in Patent Document 1, the functions of the furnace body with a vertical and cylindrical shape are basically divided into three regions along the vertical direction from the viewpoint of function. In other words, in the lower section of the furnace, there is formed a high temperature combustion zone having a coke bed in which the coke is deposited; above this high temperature combustion zone, a waste layer is formed; and in the upper section of the furnace body above the waste layer, a freeboard section having a large space is provided.
With such a waste gasification melting furnace, in the respective three regions mentioned above, oxygen-containing gas is blown into the furnace. The high temperature combustion zone in the furnace lower section is provided with amain tuyere; through a gas feeding pipe which is connected to the main tuyere, oxygen rich air is blown into the furnace; and in the high temperature combustion zone, the coke charged and deposited in the coke bed is burned to provide a melting heat source for melting the thermal decomposition residue of the waste. In addition, the waste layer is provided with a sub-tuyere for blowing air to slowly fluidize the waste which has been charged and deposited, and thermally decompose and partially oxidize the waste. Further, the freeboard section is provided with a third-level tuyere for blowing air to partially burn a part of the thermal decomposition gas (combustible gas) generated with the waste being thermally decomposed, and thus to maintain the inside of the furnace at a prescribed temperature.
Thus, the waste gasification melting furnace of shaft furnace type is a provision which, within a single furnace, can perform both the thermal decomposition gasification treatment and the melting treatment of the waste as it falls in the furnace. The charged waste is thermally decomposed, thereby gas and residue being generated. In the lower section of the furnace, by blowing oxygen rich air from the main tuyere, the coke in the coke bed is burned, a high temperature combustion zone being formed, and the thermal decomposition residue of the waste is melted to be discharged as slag and metal. The coke bed causes the oxygen rich air from the main tuyere and the high temperature gas generated on the basis of the coke combustion to be passed through the voids produced between lumps of coke, and also functions as a high temperature fire grate which passes the molten slag and metal. The high temperature gas generated by the coke combustion in the high temperature combustion zone heats the waste in the waste layer formed above the high temperature combustion zone, the waste being thermally decomposed with the air being blown from the sub-tuyere, and the gas containing combustible gas generated by such thermal decomposition rises in the waste layer, passing through the freeboard section, to be discharged from a discharge flue provided in the upper section of the furnace into a secondary combustion chamber outside the furnace. The gas, containing a large quantity of combustible gas, is burned in the secondary combustion chamber, the heat being recovered at a boiler to generate steam, which is used for electric power generation, or the like. The gas discharged from the boiler is removed of relatively coarse dust particles by means of a cyclone separator; then is cooled by a temperature lowering apparatus; is removed of noxious gas through reaction with a harmful substance removing agent; and is subjected to an exhaust gas treatment, such as that for dust removal with a dust collector, being dissipated to the atmosphere from a smoke stack.
With such a waste gasification melting furnace, a coke bed in which coke is deposited is formed in the furnace bottom section, and the coke is burned to provide a heat source for melting the thermal decomposition residue, however, in recent years, there has been a demand for reducing the consumed quantity of coke to cut the carbon dioxide emission amount.
Then, it has been studied that a fuel gas, such as a natural gas, propane gas, and combustible gas generated by thermal decomposition in a waste gasification melting furnace, is utilized as an alternative to a part of coke, and for example, in the Patent Document 2, it is proposed to blow a fuel gas together with oxygen rich air from a main tuyere (blast tuyere).
In the Patent Document 2, a tuyere for feeding oxygen rich air into the furnace is provided such a way that the distal end of the tuyere is introduced deep into the furnace to be positioned in the coke bed. The fuel gas is supplied into a gas feeding pipe connected to the tuyere, being blown into the coke bed from the distal end of the tuyere together with the oxygen rich air to be burned, and the combustion heat of the fuel gas is utilized as a part of the melting heat source in the coke bed.